The trend in automotive electrical systems has always been towards more power and higher voltages. At this time, an element of the trend involves the combination of the alternator and starter into a single IC engine driven unit. This combined starter/alternator can be driven either directly on the crankshaft of the IC engine as a part of the flywheel, on one end, or the balancer, on the other. Alternatively, the starter/alternator can be mounted for gear, belt, or chain drive from the crankshaft along with other IC engine driven components (i.e., waterpump/A/C compressor/power steering pump, etc.)
The starter/alternator has become more powerful not only for increasing power (current and voltage) but also for more rapid and more frequent starting cycles of the IC engine as enhanced operating efficiencies are sought. In pursuit of these goals, the starter/alternator has become more sophisticated in its control systems and its responsiveness to system requirements for both starter functions and generating functions.
In older systems where the alternator and starter function were performed by separate devices, the need to determine the optimal moment to transition from a starter function to an alternator function did not exist. Rather, the vehicle operator, relying on vehicle familiarity and overall driving experience, actuated the starter until engine startup was perceived. To limit the possibility of damaging the starter from overspeed, a special one-way disengaging drive was sometimes employed.
The alternator or generator was connected into older systems by virtue of a regulator that either accepted charge into the system to meet an electrical load, or for battery charging, or kept the alternator out of the system altogether as necessary. The alternator or generator in older systems did not change function from a starter to an alternator or generator.
In more modern vehicles with combined starter/alternator devices, a need exists to transition the device from starter to alternator/generator function at an appropriate moment following IC engine start-up. If the transition occurs too quickly, the IC engine may not, in-fact, have started. If it occurs too slowly, damage to the starter/alternator device may occur either directly or to the driving/driven mechanical connection between the IC engine and the device. In either case, premature wear and/or replacement is the likely outcome.